The invention relates to a method for applying brazing material to metal structures, in particular honeycomb bodies for exhaust gas catalytic converters, wherein the brazing material is applied in powder form and is firmly held on the metal structure by an adhesive material acting as a bonding agent, binder or adhesion promoter.
Applying brazing material to metal structures, such as metal catalyst carriers for exhaust gas catalytic converters, is a key problem in the effectiveness of the manufacturing process for such catalyst carriers or for the later strength and durability of the carrier itself.
It is known to apply an adhesive or binder to the metal structures to be joined, when brazing material is applied. The brazing material is in powder form and is either already mixed beforehand with the adhesive or binder or applied to them later.
German Published, Non-Prosecuted Patent Application DE 29 24 592 A1, corresponding to U.S. Pat. No. 4,381,590, describes a method for producing a carrier matrix for a catalytic reactor used for cleaning the exhaust gas of internal combustion engines in motor vehicles. In that method, the corrugated and smooth sheet steel layers are brazed together with spot brazing or full surface brazing, which is carried out by applying a brazing material application to the steel plates, particularly in strips, for instance in the direction of the corrugations of the corrugated steel sheets, or at right angles thereto. Brazing of all of the layers to one another takes place through the use of ensuing heating of the matrix. Liquid brazing paste or powdered brazing material is applied with a binder. The application of the brazing medium or binder is carried out by various methods, such as immersion, pressing in, injection into the end surfaces of the honeycomb body, etc. In order to assure uniform brazing at all of the contact points between the sheet steel layers, an excess of brazing material or binder is generally employed.
In European Patent 0 049 489 B1, a method for producing a carrier matrix for an exhaust gas catalytic converter is described. In that method, the smooth and corrugated metal sheets to be provided with a brazing material application, which are wound to make a carrier matrix for an exhaust gas catalytic converter, are first coated with a pressure-sensitive adhesive which evaporates without residue at the brazing temperature and have a surface on which a single-particle layer of dry brazing powder is applied after the drying. With the applied brazing powder, the steel sheets are then wound up and brazed to one another. A disadvantage in that method may be that the grains of brazing material will be located between the sheet-metal layers to be joined, so that when the grains of brazing material melt an undesirable gap can be created, which is detrimental to the outcome of the brazing.
As a remedy therefor, International Patent Application WO 89/11938, corresponding to U.S. application Ser. No. 08/054,161, filed Apr. 26, 1993 and 08/467,591, filed Jun. 6, 1995, describes a method for applying glue and brazing material to a metal catalyst carrier body, and an associated apparatus. In that method, the metal sheets, before being wound up or layered, are coated in the regions to be brazed with an adhesive or binder, in particular with a pressure-sensitive adhesive. Next, the sheets are wound or stacked to make a honeycomb body, and only then is the body acted upon with brazing powder. As the sheets are further transported after being coated with adhesive, and as the catalyst carrier body is being wound up or stacked in layers, or in other words in all of the handling or transport steps to be provided between the various method steps, the adhesive must be kept in a sticky to liquid state, and as a result it is possible for the adhesive to escape or run out of the catalyst carrier body, or at least for the machine parts that come into contact with the adhesive to become contaminated thereby and in the final analysis become vulnerable to malfunction.
A method and an apparatus for applying brazing material to a metal honeycomb body are also described in International Patent Application WO 93/25339, corresponding to U.S. Pat. No. 5,431,330. In that method, if no rolling oil is present on the surfaces from the manufacturing process, a thin layer of rolling oil is then first applied, which is then thermally treated in order to remove highly volatile components. The regions of the sheet-metal layers that are to be provided with brazing material are put into contact with an aqueous solution of a surfactant, and as a result they form a layer that temporarily acts as an adhesion promoter for brazing powder. After that, the sheets are wound up or stacked into honeycomb bodies. That step is followed by putting the honeycomb bodies into contact with brazing powder. The brazing powder sticks to points where residues of rolling oil and surfactant have come together. Although that method achieves economical use of brazing material, nevertheless the adhering brazing material must also be fixed with an adhesive or binder in an additional work step, to prevent it from trickling out later when transported from place to place.
It is accordingly an object of the invention to provide a method for applying brazing material to metal structures with an adhesive material having different states, which overcomes the hereinafore-mentioned disadvantages of the heretoforeknown methods of this general type, which employs an adhesive material for firmly holding brazing powder to regions that are to be brazed in metal structures to be joined together having an adhesive strength that can be varied during a production process so that a firm holding of brazing powder can be varied, which averts contamination and blocking of machines or machine parts and which cannot unintentionally flow away from a location where it is applied between the various method steps.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for applying brazing material to metal structures, in particular honeycomb bodies for exhaust gas catalytic converters, which comprises applying powdered brazing material on a metal structure in association with a production process of the metal structure; and firmly holding the brazing material on the metal structure with an adhesive material acting as a bonding agent, binder or adhesion promoter, having little or no adhesive action during a portion of the production process of the metal structure and having an ability to be subsequently intermittently adhesively activated.
In accordance with another mode of the invention, an adhesive material that is temporarily not adhesive or only slightly adhesive after application and is activatable later, in particular during the production process of the metal structure, or in other words during the various method steps required for the production process and that can assume at least two states which are different in terms of its adhesive action, is employed to firmly hold the brazing powder. One state is a less adhesive state, the other state is a more strongly adhesive state, the states of the adhesive material can be varied as a function of external conditions, such as the temperature, and in particular can be varied reversibly. An adhesive material of this kind that accomplishes its (reversible) states which can be converted to one another by the influence of light, ultraviolet, or other radiation, chemical action or other external factors, may also be used.
In accordance with a further mode of the invention, in a first exemplary embodiment the adhesive material has two aggregate states, namely a solid and a liquid state, and the solid state of the adhesive material can be converted into the liquid state or vice versa within an allowable temperature range that occurs during the production process.
In accordance with an added mode of the invention, a wax which is solid at a normal temperature of approximately 0xc2x0 C. to 50xc2x0 C. and is liquid at higher temperatures is employed as a particularly preferred adhesive material.
In accordance with an additional mode of the invention, in a likewise highly preferred exemplary embodiment, the adhesive material is a hot melt adhesive, which has no or only slight adhesive properties at its surface at temperatures below its melting temperature, or in other words approximately 200xc2x0 C., but develops a strong adhesive action above its melting temperature.
In principle, the adhesive material according to the invention can advantageously be used in all known brazing material application methods in which until now adhesives, binders, pressure sensitive adhesives or adhesion promoters were used.
In accordance with yet another mode of the invention, the adhesive material can be applied before, during or after the winding or stacking of a honeycomb body, and this can be carried out by immersion, spraying, painting and winding-in of adhesive material foil, and so forth.
The adhesive material that changes its state is especially advantageously usable in a method according to International Patent Application WO 93/25339, corresponding to U.S. Pat. No. 5,431,330. In that method, the sheet-metal layers, which as a rule are provided with a rolling oil as a result of a rolling process, are thermally degreased upon being heated, in order to obtain largely grease-free surfaces. The removal of at least highly volatile greases from the surfaces is favorable in some adhesive materials, so that the adhesive material can be applied to the still-hot sheet metal layers without the danger that the adhesive material will fail to adequately bond to the surface of the sheet-metal layers.
In accordance with yet a further mode of the invention, the adhesive material is applied to intended regions. Depending on the intended application and strength required, the application regions of the adhesive material may be larger or smaller and/or may be continuous or discontinuous. This may preferably be carried out through the use of a stick of adhesive material that is in the solid state. The adhesive material becomes liquid upon contact with the hot sheet metal and forms a liquid application. In some embodiments of the method, it may be necessary for the sheet-metal layers to be cooled down separately prior to the winding or stacking, which causes a conversion of the liquid pressure-sensitive adhesive application into a less adhesive state. However, the embodiment of the method may preferably also be carried out in such a way that in the ensuing handling or transport steps prior to the winding or stacking, the temperature lowering is adequate for the adhesive material to become less adhesive and in particular solid. This has the advantage of ensuring that in later processing steps, the adhesive material can no longer escape from the location where it is applied, and the danger of contamination or blockage of machines or machine stations will not ensue. In the cooled down, less adhesive state, the sheet metal layers provided with the adhesive material are then wound, intertwined or placed in stacks to make a honeycomb body. In the process the contact regions between the structured sheet metal layers with the layer of adhesive material between them rest on or slide past one another without sticking together. If reheating of the honeycomb body is then carried out up to a temperature at which the adhesive material is again more strongly adhesive, in particular liquid, then the adhesive material will collect especially in the gaps in the vicinity of the regions to be brazed. In this more strongly adhesive state of the adhesive material, the honeycomb body or portions thereof are acted upon with brazing powder, which remains stuck to the locations provided with adhesive material without difficulties. The stronger the influence of the external conditions, and preferably the higher the temperature to which the adhesive material is exposed, the more adhesive the adhesive material becomes. It is thus even possible with the method according to the invention to control the quantity of brazing powder taken up by the adhesive material within certain limits, depending on the desired strength, and also to influence the spatial distribution of the brazing powder at the locations actually to be brazed, and in particular to cause it to collect in nip regions.
In accordance with yet an added mode of the invention, in the case of some adhesive materials, in particular hot-melt glues, the possibility also exists of applying them to a still-existing film of rolling oil and achieve adhesion to the metal plates. The rolling oil may be used as well to make ensuing steps or winding or stacking easier. The finished honeycomb body is then degreased thermally for the first time. The adhesive remains behind, and in its then liquid state takes up brazing powder in a brazing material application step that is also carried out later at elevated temperature. Proceeding in this way is more favorable from an energy standpoint than thermal degreasing prior to the stacking or winding of the honeycomb body, and a subsequent brazing with still-warm honeycomb bodies can optionally be begun, thereby further saving energy.
In accordance with yet an additional mode of the invention, the adhesive material is applied to desired locations of the sheet-metal layers in a manner which is known per se in its more strongly adhesive, and in particular liquid, state.
In accordance with again another mode of the invention, the sheet-metal layers are wound, intertwined or stacked while the adhesive material is in a less adhesive, in particular solid, state.
In accordance with again a further mode of the invention, there is provided a method of producing honeycomb bodies, especially for exhaust gas catalytic converters of motor vehicles, that are wound, intertwined or stacked from structured sheet-metal layers, and more specifically catalytic converters disposed near the engine, which are needed for reducing pollutants during cold starting and must have an especially high brazing strain. In order to apply the brazing powder to the adhesive material, this material is first put into its adhesive, and in particular liquid, state, and the honeycomb body can then be put entirely or in certain portions into contact with brazing powder, which then remains stuck to the adhesive material at the points involved, with the adhesive material being essentially in the liquid state.
In accordance with again an added feature of the invention, the adhesive material is chosen in such a way that in its more strongly adhesive and in particular liquid or pasty state, it has such high viscosity and adhesion to the metal structures that it collects particularly in gaps and nips to be brazed, at points o f contact of the metal structures, then precisely there the brazing powder required for the brazing process can be bound without using up an unnecessarily large amount of brazing powder. The overall result attained is that an optimal adaptation to desired bonding properties when a brazed metal honeycomb body is produced is possible not only by way of the choice of the adhesive material but also by the purposeful variation of external conditions upon imposition of the brazing material.
In accordance with again an additional feature of the invention, there is provided a special variant of the method which includes imposing hot brazing powder on a honeycomb body provided with adhesive material, the hot brazing powder, wherever grains of brazing occur, briefly converting the adhesive material into its more-adhesive state and then remaining stuck. In that case, heating the entire honeycomb body is unnecessary.
In accordance with a concomitant another mode of the invention, an adhesive material is used that is applied in a first, for instance chemically dissolved state, next it changes into a second state in which it is not adhesive or is only slightly adhesive, until the honeycomb body has been completely wound, intertwined or stacked, and after that, the adhesive material is converted into its more-adhesive state, which is carried out by the influence of temperature or other external provisions, including the action of steam.
The actual brazing process after a honeycomb body is prepared in accordance with the invention m,ay include brazing in a brazing furnace, preferably in a high-temperature vacuum brazing process, in which the adhesive material evaporates essentially without residue by the action of vacuum and/or temperature. Possible residues, such as slight quantities of carbon or the like, do not impair the outcome of the brazing.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for applying brazing material to metal structures with an adhesive material having different states, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.